Shock absorber units which are provided with means for an inductive length measurement thereof are generally known, in particular from U.S. Pat. No. 4,502,006, which discloses how a cylindrical electric coil is disposed coaxially around a shock absorber housing, which housing is axially movable in the coil when there is a change of length of the shock absorber unit. When there is a change in the position of the housing relative to the coil, the electrical properties of the coil change. The phase difference between the alternating current through and the alternating voltage over an excited coil is a measure of the length of the shock absorber unit at the frequency used.
A drawback of the conventional measuring circuits is the sensitivity of the measurement to the temperature of the coil, since the electrical resistance of the coil is dependent on temperature. It is therefore necessary to take an additional measure for temperature compensation, which measure according to EP-A-0 466 604 consists of incorporating a reference resistor in the length measuring circuit. However, this gives only local information on the temperature, which information is therefore not a reliable reflection of the current temperature at the position of a measuring coil.
The accuracy of the length measurement is also adversely affected by the temperature dependence of the electric and magnetic properties of the casing of the shock absorber unit movable inside the coil.
The object of a measurement of the length of a shock absorber unit is generally to gain an idea of the load situation (or, of course, the relative position of components) of the wheel suspension of which the shock absorber unit forms part, or to put it in more general terms: the length of the shock absorber unit is a measure of the force absorbed by the corresponding wheel suspension.
A disadvantage of the shock absorber unit length measuring circuit according to the prior art is the limited band width as a result of the measuring principle used. A narrow-band measurement of the length of the shock absorber unit rules out certain important applications of such a measurement, in particular use of the measurement of the wheel suspension load condition by means of the shock absorber unit length measurement for controlling functions improving ride or riding safety, for example quick-acting active spring systems, anti-lock braking systems etc. Such applications require measurement of the speed or the acceleration of a length change of a shock absorber unit, and therefore require a great band width.